Rising oil prices, unstable supply and the demand for sustainable environmental friendly energy sources has increase the interest in research and development of bio-energy sources such as bio-ethanol. Carbohydrates are a natural resource commonly available as lignocellulosic biomass that can be hydrolyzed into sugars to be further converted via fermentative or thermochemical processes into useful products [1]. Among the important products that can be derived are ethanol (cellulosic), butanol and similar advanced fuels, platform chemicals such as acetone, furfural, levulinic acid, gamma valerolactone and bioplastics such as polyhydroxy butyrates or valerates [1-3]. These products are a substitute for fossil fuels or starch based carbohydrates, thus providing an alternate sustainable resource. The plastics are biodegradable and thus are beneficial to the environment in comparison to petrochemicals and their derivatives [4]. Cellulosic biomass is a promising material for bio-energy that avoids the usage of corn and other food grains and thus avoids the necessity of competing with edible sugars.
One of the biggest markets using cellulosic biomass is the pulp and paper industry. The global production of paper and paperboard was 403 million tons in 2011. This amount is about 30% of the industrial round-wood. The recycling rate of paper has been gradually increasing from 50% in 2007 to 53% in 2011. North America now has the highest recovery rate (64% in 2011), followed by Europe (58%) and the Asia-Pacific region (48%) [5]. This process of recycling pulp and papers is to reduce cost and to have a sustainable environmental policy. [6-8].
Repeated recycling of pulp decreases the length of fibers which become shorter and stiffer, losing their ability to bond within the paper sheet. At a certain stage, their net contribution to the sheet becomes negative and they need to be rejected. These short fibers known as fines are recovered from the wastewater stream and typically sent to landfills. The solid residue can also be applied for land use or animal bedding [9-12]. However, the fines can be a very useful resource for sugar production because they are predominantly composed of cellulose which could be converted into glucose and other monomeric sugars. Currently, some paper companies pay $25 to $80/(wet) ton for disposal of the fines [9, 13, 14]. Besides their cost advantage, the supply of fines from paper mills is fairly homogeneous and thus there is minimal influence of seasonal or weather related supply challenges compared to other agricultural biomass [6, 8].
A number of different processes including incineration, gasification and pyrolysis may be used for treating this waste fines stream [10]. However, given their energy consumption and complex processes, direct hydrolysis of the cellulose into sugars can be particularly attractive due to the simplicity of the process and ready use of the sugar solution after concentration [15]. These sugars can be used as a feedstock for conversions into biofuels and bioplastics such as polyhydroxy alkanoates or into platform chemicals such as succinic acid, lactic acid, levulinic acid and furfurals [1, 16-18].
Of the varieties of papermill fines rejects, those from recycled pulp mills using old corrugated cartons are particularly important. Some modern OCC mills find that rejecting ‘inactive’ fines into the waste stream can be more profitable than using them in the manufactured product, particularly recycled linerboard. The reject stream thus contains higher cellulosic fines contents and typically lower minerals than deinked pulp rejects in the waste streams of fine papers or tissue mills.
Lignocellulosic materials are excellent sources for energy products, platform chemicals and bioplastics. Sugars produced by the degradation of carbohydrate polymers can be fermented into ethanol and butanol as energy sources. Sugars and cellulose degradation compounds can serve as platform chemicals in the production of bulk chemicals and they can also be used as feedstocks for microbial production of plastics such as polyhydroxy alkanoates (PHA).
The waste stream from recycled paper mills contains cellulosic fines and also particles of mineral origin, typically clay or calcium carbonate from the fillers and coatings used in the waste paper. The cellulosic fines are easily hydrolyzable by either acid or enzymatic processes. In the enzymatic process, a cocktail of cellulose enzymes acts progressively and sequentially to open up the cellulose crystalline structure and depolymerize it, producing monomeric sugars. The sugars are primarily glucose and certain other common hexoses which are fermentable into ethanol, butanol or other products, leading to bioplastics such as polyhydroxy alkanoates (PHA).
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